Streptococci contain numerous toxic substances which may be secreted or be a part of the cellular structure, such as the peptidoglycan as part of the cell wall. Streptolysin S is a very potent hemolysin and membranolytic agent which is formed from an intracellular precursor. An objective is to purify SLS and precursor to determine a potential role in streptococcal disease. The development of arthritis in rats after injection of cell walls (peptidoglycan) is being studied to determine the influence of genetic background on the development of such disease.